Recently, there has been developed a computer based system for scanning negative film images, such as a collection of pictures of school children, to produce digital image signal data that is composited by the computer into page formats. The composited digital image signal data is then output to a suitable digital image printer to generate positive hardcopy prints of the composited images. In a straightforward application, the image signal data is applied to a color thermal printer wherein the images are generated by the application of color dyes onto the print material. Although high quality color prints can be produced in this manner, it is known to be more cost effective to produce multiple color prints by means of photographic processes. It has previously been proposed to take advantage of more cost effective photographic processes for the production of multiple copies by utilizing a thermal printer to dye print a negative image onto a transparent substrate material to produce a pseudo-negative that is then used to optically expose the image onto negative-working photographic print paper which can then be processed using conventional photographic processes to produce the resultant positive print image. See commonly assigned U.S. Pat. No. 5,231,506 --Manico entitled "Generation of Hard Copy Color Photo Reproductions from Digitally Created Internegative", issued Jul. 27, 1993.
While quite effective for the purpose, such a pseudo-negative method requires the use of a thermal printer which not all photographic labs possess. However, many potential users of the computer compositing machine described above already have in their photographic labs a film recorder, also referred to as a film writer, which is a device typically used to write digital images to positive slide film such as Ektachrome film produced by Eastman Kodak Company. A known use of such devices in photographic film labs involves the application of computer generated digital image signal data representing a desired graphical image to the film recorder wherein the digital image signal data is converted to an optical image by means of a cathode ray tube (CRT) for exposure onto a positive slide transparency film. The thus produced slides are commonly used as visual aids used in conjunction with oral presentations. Since film recorders are already commonly found in photographic labs, it is highly desirable to have a way in which composited image data derived by digital scanning of negative images can be converted into positive prints via the medium of the film recorder to facilitate the making of multiple prints in a cost effective manner from the digital image signal data. Thus, the objective is to use the film recorder to produce a negative film image from which the multiple positive prints can then be produced on conventional photographic print paper.
In trying to write an image directly to negative film such as Vericolor III film (Eastman Kodak Company), which is a camera negative film, it was discovered that a serious problem existed with this straightforward approach. It was found that, with a CRT-based film recorder, it was not possible to expose the film to a degree sufficient to achieve a density range (D.sub.max -D.sub.min) in the resultant negative image which would give an acceptable positive print. As the CRT exposure (image contrast) was increased, the film D.sub.max increased as desired, however, the film D.sub.min also increased by approximately the same amount, resulting in a nearly constant density range in the negative film images.
The inability to achieve a satisfactory density range on the negative film is attributed to difficulty of exposing negative film with a CRT produced image in a film recorder. Negative films have a very wide exposure latitude due to their inherently low gammas of approximately 0.65 in the case of Vericolor III (Eastman Kodak Company) camera negative film. Thus, it takes a rather large exposure of the film to develop the required density range in the film image (approximately 1.5 D) needed to produce an acceptable tonal range in the positive print. It is believed that attempts to increase the density range in the negative film image by increasing the contrast on the CRT image caused an increase in flare in the lens of the camera used for focusing the CRT image on the film in the film recorder. Additionally, the increased brightness of the CRT image is believed to cause increased internal reflection within the CRT faceplate. The combined effect is to increase the D.sub.min level in the negative image as contrast is increased in the CRT image in the attempt to increase the density range in the negative image.
It is therefore an object of the present invention to be able to produce a negative film image in a film recorder with adequate density range to achieve an acceptable positive print from the negative.